The goal of this program is an understanding of the factors that influence transcription during heterocyst differentiation in the cyanobacterium Anabaena. We have already purified and characterized the vegetative cell RNA polymerase and cloned most of the genes encoding its subunits, including the major sigma factor. We also have cloned genes for two additional sigma factors and for two proteins that regulate heterocyst differentiation. We will continue to define promoters specific for different cell types or for different stages of differentiation by deletion and point mutations, identify binding sites for transcription factors, construct suicide vectors for the selection of mutants unable to activate transcription of specific genes, design an expression system for Anabaena that allows control of transcript level in response to a small molecule, identify phosphorylated proteins that increase or decrease in response to environmental cues, and construct a high resolution physical map of the Anabaena chromosome. The map will permit the assignment of sets of genes with common regulation based on hybridization with a grid of cosmids that span the map. We are also interested in the origin and function of sets of short tandemly repeated sequences discovered at many locations in the chromosome, both following reading frames and, in one case, within the reading frame of an expressed and essential gene.